This disclosure is directed to a pulseless constant rate pumping system. Constant rate pumps are often required in many circumstances. For example in a refining process it may be necessary to inject a minute quantity of a trace constituent into a vessel against a wide range of back pressures including low to high pressures. The apparatus of the present disclosure is directed to a pump which provides such an output, namely, a constant rate of flow which is pumped at a specified pressure without pulsations in the flow rate depending upon the type of the connective tubing.
There have been attempts in the past to provide various and sundry constant rate pumping systems. The apparatus of this disclosure is an improvement over such systems and is also an improvement over the constant rate pumping system disclosed in Applicant's U.S. Pat. No. 4,127,360. The apparatus is an improvement in the sense that it incorporates a unique electronic system for achieving switchover between pumps of the apparatus and provides a rate of flow which is constant. The rate of flow is maintained steady and free of pulsations dependent upon system materials. For example, flexible plastic tubing can be used but it yields to pressure and hence serves as a somewhat inferior material to metal tubing. Metal conduit is however more costly and is used only when the performance required demands the expense. Heretofore multi-cylinder pumping mechanisms have found favor. They ordinarily however have a difficulty in achieving a switchover where the flow is coming from a first cylinder and thereafter additional cylinders in the apparatus. The switchover from a first to a subsequent cylinder has heretofore entailed a periodic surge. These have occurred during pressure build up and drop in the manifold which is common to the several cylinders. Pulses or surges in some circumstances cannot be tolerated. Accordingly, the apparatus of the present invention has overcome this handicap by the provision of a pumping system which is free of pressure surges when the multiple cylinders cycle in and out of operation.
The present apparatus overcomes these problems. The pumping apparatus disclosed herein is able to pump a fluid at a constant rate from a multi-cylinder apparatus where the pressure is free of pulses or surges. The apparatus utilizes an electronic system for controlling pump switchover and permits switching from one cylinder to the other in a pulseless fashion so that the resulting flow from the pumps is steady and continuous.
It is desirable in pumps of this nature to provide a differential pressure transducer which will measure small pressure changes at high pressure levels without danger of over pressuring the differential pressure transducer. Conventional differential pressure cells utilize a single sensing element located between two pressure ports to measure changes in pressure between the two ports. When the sensing element deflects from its zero pressure position, it provides a voltage output which indicates the magnitude and direction of the change. Voltages representing positive or negative pressure near zero incorporate considerable electrical noise that tends to interfere with electrical switching equipment. Since these systems respond to deviations from zero voltage, their signal must be fairly large to be far enough from the electrical noise associated with zero voltage output to be accurately read. Thus, if small pressure changes are to be sensed at high pressure levels (plus or minus 1 psig at 5,000 psig for example) a sensitive element of perhaps plus or minus 100 psig must be employed.
Obviously damage will occur to the differential pressure cell due to over pressuring one side and can constitute a safety hazard. During pumping which involves alternating pump action, each side will experience pressures ranging essentially from zero during filling or intake to as much as 5,000 psi when the particular side switches on line to the output. It is of course desirable to eliminate or minimize over pressuring of differential pressure cells so that the accuracy thereof can be maintained.